Why is Pluto no longer considered a planet?

When is a planet not a planet? And what is it about Pluto that has got astronomers fighting? Big Bang Blogs examines the sorry fate of the solar system's outcast.

Discovered in 1930 and once known as the ninth planet of the solar system Pluto is the smallest, coldest, and most distant ‘planet’ from the Sun. In 1978, American astronomers James Christy and Robert Harrington discovered that Pluto has a satellite, which they named Charon. Charon is almost half the size of Pluto and shares the same orbit. Pluto and Charon are thus essentially a double planet. Two additional moons, Hydra and Nix, were discovered in 2005.

Pluto's origin and identity have long puzzled astronomers. In the 1950s it was suggested that Pluto was an escaped moon of Neptune, knocked out of orbit by its largest current moon, Triton. However this theory has been heavily criticised because Pluto never actually comes near Neptune.

The solar system loses a planet

In 1992, astronomers began to discover a large population of small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition. This belt, known as the Kuiper belt, is believed to be the source of many comets. Astronomers now believe Pluto to be the largest of the known Kuiper belt objects (KBOs). Like other KBOs, Pluto shares features with comets, like the fact that the solar wind is gradually blowing Pluto's surface into space, in the manner of a comet.

The discovery of the Kuiper belt and Pluto's relation to it led many to question whether Pluto could be considered separately from others in its population. In short – was Pluto really a planet?

Moving the goalposts

One of the criteria for being classed as a planet is that an object must have “cleared the neighbourhood around its orbit." The Earth’s mass is 1.7 million times the mass of the other objects in its orbit. Unfortunately, Pluto’s mass is only 0.07 times the mass of other orbiting objects. Because of this, on September 13th 2006 Pluto was officially downgraded to a ‘dwarf planet.’